The Geologic History of Florida the Major Events of the ...

[Pages:32]March 25, 2009

The Geologic History of Florida--the Major Events of the Past that Formed the Sunshine State

Albert C. Hine College of Marine Science University of South Florida St. Petersburg, FL 33701

hine@marine.usf.edu

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Geologic time scale Inside Cover From: Press, F., Siever, R., Grotzinger, J., and Jordan, T.H., 2004, Understanding Earth: W. H. Freeman and Co., New York, 567 p.

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The Geologic History of Florida--the Major Events of the Past that Formed the Sunshine State Table of Contents

Preface Geologic Time

1. What is Florida? 2. Lost--Wandering Around the Globe and Finding Home (~700 to ~200 mya) 3. The Big Split--Formation of Four Oceans and the Establishment of the Florida Basement (~200 to ~160 mya) 4. The Carbonate Factory Cranks Up--Early Development of the Carbonate Platform (~160 to ~100 mya) 5. An Environmental Crisis--Drowning of the West Florida Margin (~100 to ~80

mya) 6. Clash of Geologic Terrains--Colliding with Cuba (~56 to ~50 mya) 7. Dissolution Tectonics--Sinkhole Development (~30 mya to Present) 8. Sands From the North--The Quartz Sand Invasion (~30 mya to Present) 9. Erosion in the Ocean, Marine Fertility, Huge Sharks, and The Florida Phosphate

Story (~22 to ~5 mya) 10. The Finish Line in Sight--Approaching Modern Florida--Emergence of South

Florida (~2.5 mya to Present) Acknowledgments References

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Preface

To those teachers who spark imagination and ignite fires of the mind.

To those students, regardless of age, who stoke those fires and have never lost their love to learn.

Having grown up in New England and majored in the earth sciences as an undergraduate, I was constantly amazed to learn that geology could explain the scenery whether it be the mountains resulting from some tectonic plate collision in deep time or relatively more recent features such Cape Cod, Long Island, and the countless more subtle topographic variations freshly made by the last mile-thick continental ice sheet that once covered the ground where we stood. The configuration and elevation of the Earth's surface, the plants that are distributed on this topography, and indeed, the nature of human habitation are controlled by the underlying geology.

Additionally, the common appearance of huge outcrops exposed in ever-increasing highway excavations, particularly in the more rugged parts of New England, allowed us to peer, at least short distance, into the Earth's crust to see rocks that had been contorted and folded by unimaginable and unseen forces of the past. These rocks were once many kilometers beneath the earth's surface, forever changed by intense heat and pressure, yet now they are exposed at the surface. How could that be? The Earth's geologic history was literally in our face and the mysteries of the past that the science allowed us to unravel were too amazing to have been invented in even the most imaginative of minds. So, I became hooked on the study of the Earth--the whole Earth, not just the continental land masses, but the interaction of members of the "sphere" family (lithosphere, atmosphere, hydrosphere, cryosphere, and the biosphere)--now called earth systems science.

Some years later, after doctoral and a post-doctoral work in the Carolinas, my wife and I moved to St. Petersburg, FL where I had accepted a faculty position at the University of South Florida in what was the Department of Marine Science (now a College of Marine Science). By then, I had become a student of the geology of the ocean--a geological oceanographer-- thus fulfilling a boyhood wish to study the marine realm.

So, Florida with its huge coastal ocean and coastline was a great place to start one's career in geological oceanography. But, the land geology of Florida was not in-your-face, so to speak. The highest natural point in Florida is only ~105 m (345 ft) and that is in the

Figure 1 Elevation map of Florida. The Panhandle has the highest elevations in Florida.

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panhandle (Britton Hill--northern Walton County). Britton Hill is the lowest, highest point of any of the 50 states. Overall, rocky outcroppings are relatively rare in Florida. By comparison, Mt. Mitchell in the North Carolina Appalachians is 2,025 m (6,684 ft)--the highest point east of the Mississippi River, still hardly of the magnitude or grandeur of the Himalayan Mountains or even the Rockies.

Figure P.1 Mount Mitchell in the North Carolina Blue Ridge Mountains is now 2,025 m above sea level, but once may have stood over 8,000 m above sea level. The attraction to Florida, at least for me and for many others, is the ocean. Florida, as we will see, was born from the ocean, and its geologic history lies beneath our feet, deep underground, well out of sight, and therefore out of mind to most of us. People come to Florida not to see the geologic events of the past, but the geology of the present--the coastline and its beaches, the fresh-water springs, the coral reefs, the Everglades, the swamps and the wildlife associated with these environments.

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Figure P.2A Deep light dependent coral reef on west Florida shelf, Pulley Ridge (~70 m depth). B. Manasota Key barrier island having--one of Florida's great beaches. C. Diver in one of Florida's popular freshwater spring. D. One of the many important wetlands in Florida supporting unique ecosystems. E. Space imagery of south Florida showing world famous Everglades and Keys.

So, the geologic history of Florida is a hidden secret--known only to a privileged few whose work, hobby, or both is to study Florida's geologic past.

Over the years, I thought that it was important for our marine science graduate students, particularly those interested in geological oceanography, to know something about Florida's geologic past. Thus, I developed a full-length course on the geologic history of Florida, contributed a chapter to The Geology of Florida*, and have published a number of scientific papers on the details of selected portions of Florida's geologic history.

But, this book is not meant to be a textbook for my course. I have a broader audience in mind. As my own children were growing up, I would make a point to volunteer to come into their classrooms, much to their embarrassment, and talk about oceanography, earth science, and relate it to Florida's past. I would like to think that the kids learned something. But, I was always struck by the teachers telling me that they had no idea Florida had such an interesting geologic past. As a result, I have written this book with

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those teachers in mind as well as all of those out there who have never lost their love to learn something different.

There is a great quote from the poet Yeats who said that "Education is not the filling of a pail, but the lighting of a fire" (William Butler Yeats; 1865-1939). I hope this book starts some fires. Unfortunately, much of our learning today is about filling heads (pails) with information required by standardized tests and not starting enough fires in the mind I realize that, to some, there is more here than you want to know. Others will feel that the book is not rigorous enough to use a course textbook leaving them unsatisfied by some explanations--there is simply not space to flesh out nuances or to present multiple interpretations. Rather, I have tried to provide the broad perspective and have approached this effort based upon the major events and the processes (chemical, biological, and physical) that have taken place, not just marching lock-step through geologic time explaining the land-based sequence of rock/sedimentary formations.

People come to Florida to enjoy the modern environment. And, this environment is undergoing severe stress presented by the ~18 million people that now inhabit the state. Water quality and quantity problems, waste disposal, accidental spills of toxic substances, hurricane threats, other weather extremes (floods, droughts), beach erosion, phosphate and limestone rock mining, coral-reef degradation, offshore oil drilling, live hard-bottom excavation, channel dredging, coastal wetlands and marine vegetation impacts, harmful algal blooms are all subjects that contain an important geologic environmental component. A balanced, science-based assessment of these topics under one cover is a formidable task and is best left as a follow-on effort. But, an understanding of Florida's geologic past is a necessary first step to fully address all of these issues.

In the end, Florida is intimately and ultimately tied to global geologic events that synergistically link the atmosphere, ocean, crust and all life contained therein. Florida's geologic history is not just a local phenomenon, but one that is interconnected to Earth's interior and earth's surface events. Plate motion, ocean circulation, climate, biotic evolution, and sea-level changes are global phenomena whose integrated signal may appear differently in different places around the globe at different times in the past. One of the great lessons of Earth science is that any specific location on Earth is always linked to a much larger framework. It is our task as readers and interpreters of Earth history to recognize and understand that linkage.

So, take what you can from this effort. If you crave more, I have included some key references at the end that may provide the required detail. More importantly, have fun learning about this amazing sequence of geologic events that have conspired to bring Florida to its present day. As I tell my students--if it isn't fun, it isn't worth doing.

*Randazzo,A.F., and Jones, D.S., 1997, The Geology of Florida: Gainesville, FL, University Press of Florida, ISBN 0-8130-1494-4, 327 p.

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Geologic Time and Metric Units

"We find no vestige of a beginning, no prospect of an end." James Hutton's remarks to the Royal Society of Edinburgh in 1788 concerning the age of the earth. Playfair later commented..., "the mind seemed to grow giddy by looking so far into the abyss of time." (Transactions of the Royal Society of Edinburgh, vol. V, pt.

III, 1805, quoted in Natural History, June 1999.)

Students new to geologic time and the Earth's great antiquity are amazed by the enormity of scale (see Geologic Time Scale on inside cover). Geologists toss around 10's, if not 100's of millions of years as if we were talking about something that happened last week or last month. In a geologic sense, we are. Since the age of the Earth is about 4.55 billion years old, what's a few million years here and there?

To a "visual" person, converting time to distance may provide a perspective on "deep" time. For example, let's assume that 1 millimeter (.001 meter; ~thickness of dime) equals 1 year. Then, 1 million years (13,333 consecutive human lifetimes @ 75 yrs each; 40,000 consecutive human generations @ 25 yrs each) equals 1 million millimeters (mm) or 1 kilometer (km). One kilometer is about .62 statute miles (not nautical). In this book I use Ma to mean millions of years as in 100 Ma and mya to mean millions of years ago--same with thousands of years (Ka) and thousands of years ago (kya).

Now, how about a billion years (Ga)? Again, this is one billion millimeters, which is 1,000 km (~620 miles). So, the age of the earth at 4.55 Ga, if plotted on a graph would be 4,550 km (~2,852 miles) away from the zero point (today; 0 years) if 1 yr = 1 mm. In this book, we travel back to about 700 Ma to consider Florida's geologic history with most of the action occurring during the last 200 Ma (about 4% of the total age of the Earth). In this sense, Florida is but a toddler.

During some periods of geologic history, little may have happened during the passage of 10's of millions of years. At other times, a short-lived, but spectacular event such as the large meteorite striking the Earth ~65.5 mya (defining the famous K/T boundary) can permanently alter earth history even though the immediate event lasted no more than a few seconds. So, some events take longer than others. The amount of time consumed during each event is not particularly relevant, but the consequences are significant. As a result, the chapters below do not each represent an equal amount of geologic time. But, each is important to the ultimate development of Florida.

Metric units beyond this point will be used only. Having grown up with English units as part of my DNA, it is still hard to visualize some measurements expressed in metric-- particularly temperature. Even after 35 years as a scientific researcher, I still don't know if I should wear a sweater outside if the temperature is 20oC. But, it is a metric world out there, and for sure, scientists in most other countries (including England) use metric. So, we all have to get used to it--the sooner the better.

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